Three known types of volumetric flowmeters include turbine meters, vortex shedding flowmeters, and fluidic, oscillating jet-type flowmeters. In comparison to turbine meters, the fluidic oscillating jet flowmeter offers greater reliability and insensitivity to contamination since the fluidic flowmeter has no mechanical moving parts. The fluidic flowmeter also has a much better frequency response characteristic than a turbine flowmeter since it does not have a major inertial lag function as associated with the rotating turbine flowmeter. Most known turbine flowmeters and fluidic jet oscillating flowmeters are similar in terms of flow turn down ratio, signal amplitude and signal quality, even though the turbine flowmeter in many applications produces less pressure drop in the fluid flow being measured than previous fluidic oscillating jet flowmeters.
Vortex shedding flowmeters are characterized by a bluff body introduced into the fluid flow to be measured to create an alternating series of vortices, externally of the bluff body, that propagate downstream from the bluff body. Such type flowmeters are susceptible to vibration and are very sensitive to system pressure pulsation and flow disturbances, normally requiring a significant amount of electronic signal conditioning to produce a clean output signal. In comparison, a fluidic oscillating jet flowmeter is relatively unaffected by external vibration and may be designed to provide built-in isolation from system pressure pulsations and/or external vibrations.
Examples of flowmeters utilizing fluidic negative feedback oscillators may be found in U.S. Pat. Nos. 4,508,127 and 4,949,755. The latter '755 patent describes an improved fluidic volumetric flowmeter which utilizes a plurality of parallel flow paths, each having a restrictor therein, which flow paths together carry the entire volume of fluid flow being measured. As noted, such structure does have the drawback of producing a relatively significant pressure drop in the fluid flow being measured, which pressure drop represents an unrecovered energy loss.
Other types of volumetric flow sensors operate on the principal of measurement of static pressure differentials and suffer from the inherent difficulties associated with such static pressure measurement systems including insensitivity, limited range, mechanical failures, and limited accuracy. Examples of these include fixed restrictors or orifices generating a continuous pressure drop in the fluid flow being measured, and venturi type pressure pickup measuring devices. The venturi type device does afford the advantage, in comparison to fixed orifice type restrictors, in that a significant portion of the pressure drop occurring through a venturi orifice is subsequently recovered in the downstream diffuser or pressure recovery section of the venturi nozzle.